[lib] Add COMMUNITY_ROOT to NewThreadRequest
[lib] Add COMMUNITY_ROOT to NewThreadRequest

Summary: Note that I haven't added it to `newThreadRequestInputValidator`, so it's not possible for API requests to create a new `COMMUNITY_ROOT`. Only programmatic calls to `createThread` in the server codebase can do it.

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1240

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal